TARGET_DEFINED subscription mode support with JTI

Junos OS  adds support for TARGET-DEFINED mode for subscriptions made using gRPC Network Management Interface (gNMI) services.

Using a gNMI subscription, an external collector stipulates how sensor data should be delivered:

• STREAMING mode periodically streams sensor data from the DUT at a specified interval.
• ON_CHANGE mode sends updates for sensor data from the DUT only when data values change.
• Newly supported TARGET_DEFINED mode (submode 0) instructs the DUT to select the relevant mode (STREAMING or ON_CHANGE) to deliver each element (leaf) of sensor data to the external collector.

When a subscription for a sensor with submode 0 is sent from the external collector to the DUT, the DUT responds, activating the sensor subscription so that periodic streaming does not include any of the ON_CHANGE updates. However, the DUT will notify the collector whenever qualifying ON_CHANGE events occur.